1. Field of the Invention
The present invention relates to an antenna device including a helical antenna.
2. Description of the Related Art
Conventionally, a helical antenna is widely used as a linear antenna that has a satisfactory circularly-polarized wave property.
When a helical antenna is used alone, a directivity control is difficult. Thus, JP-A-8-789946 discloses an antenna device having an array structure in which a plurality of helical antennas is arranged on a surface of a reflecting plate.
In the antenna device having the array structure, directivity is controlled while keeping a shape of an antenna beam. Therefore, the helical antennas need to be arranged at an interval of a half wavelength of use, and it is difficult to reduce a dimension of the antenna device.